Presbyopia is an age-related loss of accommodation of the human eye in which the lens of the eye becomes less flexible. As a result, the ability to shift focus from distant to near objects is compromised. FIG. 1 is a schematic of the human eye illustrating the various distances between which an eye with normal visual acuity can focus. Ophthalmic lenses with fixed focusing properties have been widely used as spectacles and contact lenses to aid in correcting presbyopia and other vision conditions.
Ophthalmic lenses are most useful if they have adjustable focusing power (i.e., the focusing power is not static). Adjustable focusing power provides the eye with an external accommodation to bring objects of interest at different distances into focus. Adjustable focusing power can be achieved using a mechanical zoom lens. However, the mechanical approach makes the spectacle bulky and costly.
One technique commonly used by ophthalmic lenses is area division. Area division refers to bifocal, trifocal, progressive, or contact lenses that enable the eye to focus on both near and distant objects by looking through a different section of the lens. FIG. 2A illustrates the portions of the lens aperture used for near and distance vision for aspheric 210, concentric 220, and translating 230 bifocal lenses. FIG. 2B illustrates the portions of the lens aperture used for near, distance, and intermediate vision in a trifocal lens 240. As illustrated, the field of view for each type of vision (e.g., near, intermediate, distance) is generally limited to a narrow corridor.
Additionally, progressive lenses cause some distortion. Many bifocal or multifocal contact and intraocular lenses use the simultaneous vision concept, where light is equally diffracted into several orders and each order corresponds to a focal length. The light efficiency is low and the user selectively suppresses the most blurred images that are not desired for a given task.
Another type of ophthalmic lenses is mono-vision lenses. Mono-vision lenses typically have a convex-concave shape, by which different focusing power is provided to each eye, one for near and the other for distant objects. However, mono-vision lenses affect the user's binocular depth perception. These lenses would be more capable and attractive if one could change their focusing power.
Currently known techniques, including liquid crystal (LC) adaptive lens technologies, that would allow the focusing power to be changed are not suitable for use with an ophthalmic lens. For example, fluidic adaptive lenses have been demonstrated, but are not particularly effective since the shape of the liquid is sensitive to external vibrations. Known electrically controllable liquid-crystal refractive lenses incorporating convex and concave substrates require a thick layer of LC (>400 um). Optical scattering caused by the thick LC layer results in low transmission and lengthy response and recovery times.
In LC lens designs that employ a Fresnel lens substrate, the thickness of the LC lens is reduced but the lens is optically active in the electrically off-state. This design is not desirable for ophthalmic applications since a loss of electrical power resulting in near-vision correction could be detrimental if it occurs during a critical distance vision task, such as driving. Thus, known LC lenses are unsuitable for ophthalmic applications, which require high light efficiency, relatively large aperture, fast switching time, low driving voltage, and power-failure-safe configuration.